Invoking an external module function for Activity History Details (the Callback property) doesn't work

The code at issue is CRM/History/Page/Activity.php -> the run() function. This currently only works for internal (civicrm) class::method callbacks. Things we'll need to do conditionally...

• Remove the existing error trap on the require_once()

• If callback isn't a static method in a class (class::method) but
  'just' a function, tweak code to call the function
  properly - and use function_exists() for error trapping.